Many modern communications standards such as, for example, WLAN (Wireless Local Area Network), LTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access), require transmitters capable of generating wideband signals that also have high peak-to-average power ratios (PAPRs). Because a transmitter's energy consumption is determined in large part by how energy efficient its power amplifier (PA) is, it is desirable to employ a polar transmitter to transmit these high PAPR signals. Polar transmitters allow the use of highly efficient, nonlinear PAs, despite the high PAPR. However, they also require a wideband phase modulator.
Traditionally, phase modulators have been implemented within a phase-locked loop (PLL). As illustrated in FIG. 1, a PLL-based phase modulator 100 comprises a phase detector 102, a loop filter 104, a voltage-controlled oscillator (VCO) 106, and a combiner 108 through which a phase-modulating signal is “injected” into the loop. The phase detector 102 operates to compare the phase of the signal produced at the output of the VCO 106 to the phase of a reference signal of frequency fREF provided by a reference oscillator 110 and generate an error signal having a magnitude depending on the determined phase difference. The loop filter 104 performs a low-pass filtering function on the error signal, and the combiner 108 combines the phase-modulating signal with the filtered error signal, resulting in a combined phase-modulating signal which is applied to the control input of the VCO 106. Changes in signal phase introduced by the phase-modulating signal are therefore developed at the output of the VCO 106. Meanwhile, the PLL works continuously to reduce the phase difference between the resulting phase-modulated signal, which is fed back to the phase detector 102, and the reference signal provided by the reference oscillator 110.
One problem with the PLL-based phase modulator 100 is that low-frequency content in the phase-modulating signal that falls within the loop filter 104 bandwidth is compensated by the PLL and therefore not produced at the output of the VCO 106. This problem can be avoided by injecting the phase-modulating signal at the input of the reference oscillator 110, instead of at the input of the VCO 106. However, then, only the frequency content that is within the loop filter 104 bandwidth is produced at the output of the VCO 106. In other words, high-frequency modulation content tends to be filtered out. Because these two approaches produce complementary results, however, they can be beneficially combined, as illustrated in FIG. 2, resulting in what is known as a “two-point” phase modulator 200.
The two-point phase modulator 200 removes the dependency of the modulation bandwidth on the loop bandwidth of the PLL. However, the modulation bandwidth is still effectively limited by the bandwidth handling capability of the VCO 106. Many modern digital phase modulation schemes produce phase-modulating signals having abrupt phase changes (i.e., phase “shifts”), some even producing phase-modulating signals having complete 180 degree phase reversals. These types of signals contain very high frequency content. However, any practical VCO is capable of operating linearly only over a finite range of frequencies. Attempts to modulate the VCO beyond the linear range of operation can lead to substantial phase modulation errors.
High phase accuracy also requires that there be no significant gain or phase mismatch between the two modulation paths. This requirement can be difficult to satisfy, especially since the ability to match the two modulation paths depends on the modulation sensitivity of the VCO 106, which as explained above is not always a linear device.
A final problem with the two-point modulator 200 is that the reference oscillator 110 must be implemented as a VCO. In other words, two VCOs are needed. This requirement undesirably complicates the design and increases manufacturing costs.
Two-point phase modulators have been successfully implemented in GSM/EDGE (Global System for Mobile Communications/Enhanced Data Rates for GSM Evolution) and W-CDMA (Wideband Code Division Multiple Access) communications systems. However, because of the drawbacks and limitations discussed above, they have yet to be shown to be a suitable solution for WLAN, LTE, WiMAX and other developing wideband communications standards. It would be desirable, therefore, to have phase modulation methods and apparatuses that are capable of supporting WLAN, LTE, WiMAX and other wideband communications standards and at the same time are capable of providing the modulation resolution needed to satisfy the modulation accuracy requirements of these standards.